Insulators for high density circuits

ABSTRACT

A conductive system and a method of forming an insulator for use in the conductive system is disclosed. The conductive system comprises a foamed polymer layer on a substrate. The foamed polymer layer has a surface that is hydrophobic, and a plurality of conductive structures are embedded in the foamed polymer layer. An insulator is formed by forming a polymer layer having a thickness on a substrate. The polymer layer is foamed to form a foamed polymer layer having a surface and a foamed polymer layer thickness, which is greater than the polymer layer thickness. The surface of the foamed polymer layer is treated to make the surface hydrophobic.

[0001] This application is a Divisional of U.S. Application No.09/382,524, filed Aug. 25, 1999 which is incorporated herein.

FIELD OF THE INVENTION

[0002] This invention relates to high density integrated circuits, andmore particularly to insulators used in high density circuits.

BACKGROUND OF THE INVENTION

[0003] Silicon dioxide is the most commonly used insulator in thefabrication of integrated circuits. As the density of devices, such asresistors, capacitors and transistors, in an integrated circuit isincreased, several problems related to the use of silicon dioxideinsulators arise. First, as metal signal carrying lines are packed moretightly, the capacitive coupling between the lines is increased. Thisincrease in capacitive coupling is a significant impediment to achievinghigh speed information transfer between and among the integrated circuitdevices. Silicon dioxide contributes to this increase in capacitivecoupling through its dielectric constant, which has a relatively highvalue of four. Second, as the cross-sectional area of the signalcarrying lines is decreased for the purpose of increasing the packingdensity of the devices that comprise the integrated circuit, the signalcarrying lines become more susceptible to fracturing induced by amismatch between the coefficients of thermal expansion of the silicondioxide and the signal carrying lines.

[0004] One solution to the problem of increased capacitive couplingbetween signal carrying lines is to substitute a material for silicondioxide that has a lower dielectric constant than silicon dioxide.Polyimide has a dielectric constant of between about 2.8 and 3.5, whichis lower than the dielectric constant of silicon dioxide. Substitutingpolyimide for silicon dioxide lowers the capacitive coupling between thesignal carrying lines. Unfortunately, there are limits to theextendibility of this solution, since there are a limited number ofinsulators that have a lower dielectric constant than silicon dioxideand are compatible with integrated circuit manufacturing processes.

[0005] One solution to the thermal expansion problem is to substitute afoamed polymer for the silicon dioxide. The mismatch between thecoefficient of thermal expansion of a metal signal carrying line and thecoefficient of thermal expansion a foamed polymer insulator is less thanthe mismatch between the coefficient of thermal expansion of a metalsignal carrying line and the coefficient of thermal expansion of silicondioxide. Unfortunately, a foamed polymer has the potential to adsorbmoisture, which increases the dielectric constant of the foamed polymerand the capacitive coupling between the metal signal carrying lines. Onesolution to this problem is to package the integrated circuit in ahermetically sealed module. Unfortunately, this solution increases thecost of the integrated circuit.

[0006] For these and other reasons there is a need for the presentinvention.

SUMMARY OF THE INVENTION

[0007] The above mentioned problems with silicon dioxide insulators andother problems are addressed by the present invention and will beunderstood by reading and studying the following specification.

[0008] A conductive system and a method of forming an insulator for usein the conductive system is disclosed. The conductive system comprises afoamed polymer layer formed on a substrate. The foamed polymer layer hasa surface that is hydrophobic. A plurality of conductive structures areembedded in the foamed polymer layer.

[0009] An insulator is formed by forming a polymer layer having athickness on a substrate. The polymer layer is foamed to form a foamedpolymer layer having a surface and a foamed polymer layer thickness,which is greater than the thickness of the polymer layer. The surface ofthe foamed polymer layer is treated to make the surface hydrophobic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1A is a perspective cross-sectional view of one embodiment ofa conductive system of the present invention.

[0011]FIG. 1B is a enlarged view of a section of the foamed material ofFIG. 1A.

[0012]FIG. 2 is a perspective cross-sectional view of one embodiment ofa plurality of stacked foamed polymer layers formed on a substrate.

[0013]FIG. 3 is a perspective view of one embodiment of an air-bridgestructure suitable for use in connection with the present invention.

[0014]FIG. 4 is block diagram of a system level embodiment of a computersystem suitable for use in connection with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificpreferred embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that logical, mechanical andelectrical changes may be made without departing from the spirit andscope of the present inventions. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims.

[0016]FIG. 1A is a perspective cross-sectional view of one embodiment ofconductive system 100. Conductive system 100 includes substrate 103,foamed material layer 106, conductive structure 109, and conductivestructure 112. Foamed material layer 106 is formed on substrate 103, andthe plurality of conductive structures, conductive structure 109 andconductive structure 112, in one embodiment, are embedded in foamedmaterial layer 106.

[0017] Substrate 103 is fabricated from a material, such as asemiconductor, that is suitable for use as a substrate in connectionwith the fabrication of integrated circuits. Substrate 103 includesdoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures having an exposed surface with which to formthe conductive system of the present invention. Substrate 103 refers tosemiconductor structures during processing, and may include other layersthat have been fabricated thereon. In one embodiment, substrate 103 isfabricated from silicon. Alternatively, substrate 103 is fabricated fromgermanium, gallium-arsenide, silicon-on-insulator, orsilicon-on-sapphire. Substrate 103 is not limited to a particularmaterial, and the material chosen for the fabrication of substrate 103is not critical to the practice of the present invention.

[0018] Foamed material layer 106 is formed on substrate 103. Foamedmaterial layer 106 includes surface 115, foamed thickness 118, andfoamed section 121. In preparing to form foamed material layer 106, anunfoamed material layer is applied to the surface of substrate 103. Inone embodiment, the unfoamed material layer is applied using aconventional photoresist spinner to form an unfoamed material layer. Inone embodiment, the unfoamed material layer is fabricated from apolymer, such as polyimide or parylene containing silane, that iscapable of being foamed to a foamed thickness 118 of about three timesthe starting thickness of the unfoamed polymer layer. Alternatively, theunfoamed material layer is a gel, such as an aerogel, that is capable ofbeing foamed to an foamed thickness 118 of about three times thestarting thickness of the unfoamed gel layer. In still another alternateembodiment, the unfoamed material layer is formed from a material thathas a dielectric constant of less than about 1.8 after foaming andcontains silane. After curing, the thickness of the unfoamed materiallayer is preferably between about 0.6 and 0.8 microns, which is lessthan foamed thickness 118. If a final thickness of the foamed materialof 2.1 microns with a dielectric constant of 0.9 is required, then athickness less than about 0.6 microns may result in insufficientstructural strength, to support the conductive structures 109 and 112. Athickness of more than about 0.8 microns would result in a higher thandesired dielectric constant.

[0019] After the unfoamed material layer is applied to substrate 103, anoptional low temperature bake can is performed to drive off most of thesolvents present in the unfoamed material layer. If needed, the unfoamedmaterial layer is cured. If the unfoamed material layer is formed froman organic polymer, such as a polyimide, a fluorinated polyimide, or afluro-polymer, curing the organic polymer results in the organic polymerdeveloping a large number of cross-links between polymer chains. Avariety of techniques are available for curing polymers. For example,many polymers are cured by baking in a furnace (e.g., at about a 350°Centigrade (C) to about 500° C.)) or heating on a hot plate to the sametemperatures. Other polymers are cured by exposing them to visible orultraviolet light. Still other polymers are cured by adding curing (e.g.cross-linking) agents to the polymer. Preferably, some types of polymersare most effectively cured using a process having a plurality ofoperations. For example, a curing process having a plurality ofoperations includes the operations of processing in the range oftemperatures of between about 100° C. and about 125° C. for about 10minutes, processing at about 250° C. for about 10 minutes, andprocessing at about 375° C. for about 20 minutes. Preferably, a hotplate is used in performing a curing process having a plurality ofoperations.

[0020] A supercritical fluid is utilized to convert at least a portionof the unfoamed material layer into foamed material layer 106. A gas isdetermined to be in a supercritical state (and is referred to as asupercritical fluid) when it is subjected to a combination of pressureand temperature such that its density approaches that of a liquid (i.e.,the liquid and gas state coexist). A wide variety of compounds andelements can be converted to the supercritical state for use in formingfoamed material layer 106.

[0021] Preferably, the supercritical fluid is selected from the groupcomprising ammonia (NH₃) an amine (e.g., NR₃), an alcohol (e.g., ROH),water (H₂O), carbon dioxide (CO₂), nitrous oxide (N₂O), noble gases(e.g. He, Ne, Ar), a hydrogen halide (e.g., hydrofluoric acid (HF),hydrochloric acid (HCl), or hydrobromic acid (HBr)), boron trichloride(BCl₃), chlorine (Cl₂), fluorine (F₂), oxygen (O₂), nitrogen (N₂), ahydrocarbon (e.g., methane (CH₄), ethane (C₂H₆), propane (C₃H₈),ethylene (C₂H₄), etc.), dimethyl carbonate (CO(OCH₃)₂), a fluorocarbon(e.g. CF₄, C₂F₄, CH₃F, etc.), hexfluoroacetylacetone (C₅H₂F₆O₂), andcombinations thereof. Although these and other fluids are used assupercritical fluids, preferably a fluid with a low critical pressure,preferably below about 100 atmospheres, and a low critical temperatureof about room temperature is used as the supercritical fluid. Further,it is preferred that the fluids be nontoxic and nonflammable. Inaddition, the fluids should not degrade the properties of the unfoamedmaterial. Preferably, the supercritical fluid is CO₂ because it isrelatively inert with respect to most polymeric materials. Furthermore,the critical temperature (about 31° C.) and critical pressure (about7.38 MPascals (MPa), 72.8 atmospheres (atm)) of CO₂ are relatively low.Thus, when CO₂ is subjected to a combination of pressure and temperatureabove about 7.38 MPa (72.8 atm) and about 31° C., respectively, it is inthe supercritical state.

[0022] The unfoamed material layer is exposed to the supercritical fluidfor a sufficient time period to foam at least a portion of the unfoamedmaterial layer to foamed thickness 118. Generally, substrate 103 isplaced in a processing chamber and the temperature and pressure of theprocessing chamber are elevated above the temperature and pressureneeded for creating and maintaining the particular supercritical fluid.After the unfoamed material layer is exposed to the supercritical fluidfor a sufficient period of time to saturate the unfoamed material layer,the processing chamber is depressurized. Upon depressurization, thefoaming of the unfoamed material layer occurs as the supercritical stateof the fluid is no longer maintained.

[0023] The foaming of a particular material is assisted by subjectingthe material to a thermal treatment, e.g., a temperature suitable forassisting the foaming process but below temperatures which may degradethe material. The depressurization to ambient pressure is carried out atany suitable speed, but the depressurization must at least provide forconversion of the polymeric material before substantial diffusion of thesupercritical fluid out of the polymeric material occurs. Foaming of theunfoamed material layer occurs over a short period of time. The periodof time that it takes for the saturated unfoamed material layer to becompletely foamed depends on the type and thickness of the material andthe temperature/pressure difference between the processing chamber andambient environment. The specific time, temperature, and pressurecombination used depends on the diffusion rate of the gas through thematerial and the thickness of the layer of material.

[0024] U.S. Pat. No. 5,334,356, Supermicrocellular Foamed Materials,Daniel F. Baldwin et al. and U.S. Pat. No. 5,158,986, MicrocellularThermoplastic Foamed With Supercritical Fluid, Cha et al. describealternate supercritical fluid processes for foaming a material, whichare suitable for use in connection with the present invention, and whichare hereby incorporated by reference.

[0025] After completion of the foaming process, in one embodiment,foamed material layer 106 is exposed to a methane gas which has beenpassed through a plasma forming CH₃ and H radicals. The CH₃ radicalsreact with foamed material 106 at surface 115 making surface 115hydrophobic.

[0026]FIG. 1B is a magnified view of foamed section 121 in foamedmaterial layer 106 of FIG. 1A. Foamed section 121 is a cross-sectionalview of a plurality of cells 127 that make up foamed section 121. Eachof the plurality of cells 127 has a cell size. For example, cell 131 hascell size 133. The plurality of cells 127 has an average cell size. Inone embodiment, the average cell size is less than distance 130 betweenconductive structure 109 and conductive structure 112 of FIG. 1A. If theaverage cell size is not less than distance 130 between conductivestructure 109 and conductive structure 112, the microstructure of foamedmaterial 106 is not sufficiently dense to support conductive structure109 and conductive structure 112 of FIG. 1A. In one embodiment, theaverage cell size 133 is less than about one micron, and the averagecell size is less than about one micron. Preferably, cell size 133 isless than about 0.1 microns and the average cell size is less than about0.1 microns.

[0027] Referring again to FIG. 1A, conductive structure 109 andconductive structure 112 are embedded in foamed material layer 106.Prior to embedding conductive structure 109 and conductive structure 112in foamed material layer 106, photoresist is applied to surface 115 offoamed material layer 106. In one embodiment, patterns for through holesand channels are formed in the resist using a gray mask pattern.Alternatively, two levels of photoprocessing are used to define thepatterns. After photoprocessing, holes and channels are etched in foamedmaterial layer 106. A metal, such as aluminum, copper, gold, silver, ortungsten or an alloy of aluminum, copper, gold, silver, or tungsten ofsufficient thickness to fill the trenches and through holes is depositedon the surface of foamed material layer 106. Chemical mechanicalpolishing (CMP) can be used to remove the excess metal from surface 115.The process is repeated as many times as necessary to build a completewiring structure.

[0028] Conductive system 100 has several advantages. First, thedielectric constant of foamed material layer 106 located betweenconductive structure 109 and conductive structure 112 is less than thedielectric constant of the commonly used silicon dioxide insulator. So,the information bandwidth of conductive structure 109 and conductivestructure 112 is increased. Second, the surface of foamed polymer layer106 is hydrophobic, which prevents moisture from accumulating in theinterstices of foamed polymer layer 106 and increasing the dielectricconstant. Third, forming foamed polymer layer 106 from a gel has theadded advantage that a foamed gel has high thermal stability, so lowerthermal stresses are exerted on conductive structures 109 and 112.

[0029]FIG. 2 is a perspective cross-sectional view of one embodiment ofa multilayer conductive system 200. Multilayer conductive system 200includes substrate 203, foamed material layer 206, foamed material layer209, first level conductive structures 212, 215, and 218, and secondlevel conductive structures 221, 224, and 227. Foamed material layer 206is formed on substrate 203. Foamed material layer 209 is formed onfoamed material layer 206. First level conductive structures 212, 215,and 218 are embedded in foamed material layer 206, and second levelconductive structures 221 224, and 227 are embedded in foamed materiallayer 209.

[0030] Substrate 203 provides a base for the fabrication of integratedcircuits. Substrate 203 is fabricated from the same materials used inthe fabrication of substrate 103 of FIG. 1 described above. Foamedmaterial layer 206 and foamed material layer 209 are formed using theprocesses described above in forming foamed material layer 106 of FIG.1.

[0031] First level conductive structures 212, 215, and 218, in oneembodiment, are formed using conventional integrated circuitmanufacturing processes. Second level conductive structures 221 and 227,in one embodiment, are formed using the dual damascene process. The dualdamascene process is described in “Process for Fabricating Multi-LevelIntegrated Circuit Wiring Structure from a Single Metal Deposit”, JohnE. Cronin and Pei-ing P. Lee, U.S. Pat. No. 4,962,058, Oct. 9, 1990, andis hereby incorporated by reference. An advantage of the presentinvention is that it is suitable for use in connection with the dualdamascene process, which reduces the cost of fabricating multi-levelinterconnect structures in integrated circuits.

[0032]FIG. 3 is a perspective view of one embodiment of air-bridgestructure 300, which is suitable for use in connection with the presentinvention. Air-bridge structure 300 comprises substrate 303, air-bridgestructure 306, air-bridge structure 309, and electronic devices 312,315, 318, and 321. Electronic devices 312, 315, 318, and 321 are formedon substrate 303. Air-bridge structure 306 interconnects electronicdevices 312 and 315, and air-bridge structure 309 interconnectselectronic devices 318, and 321.

[0033] Substrate 303 provides a base for the fabrication of electronicdevices. Substrate 303 is fabricated from the same materials used in thefabrication of substrate 103 of FIG. 1 described above.

[0034] Air-bridge structures 306 and 309 are conductive structures.Conductors suitable for use in the fabrication of air-bridge structures306 and 309 include silver, aluminum, gold, copper, tungsten and alloysof silver, aluminum, gold, copper and tungsten. Air-bridge structures306 and 309 are surround by air, which has a dielectric constant ofabout one, so the capacitance between air-bridge structure 306 and 309is less than the capacitance between two similarly configured conductivestructures embedded in silicon dioxide. Decreasing the capacitancebetween air bridge structure 306 and air-bridge structure 309 from aboutfour to one allows the transmission of higher frequency signals betweenelectronic devices 318 and 321 and electronic devices 312 and 315. Thebandwidth is increased further by treating the surfaces of air-bridgestructures 306 and 309 to make them hydrophobic. In one embodiment amethod for treating the surfaces of air-bridge structures 309 and 312comprises creating methane radicals by passing methane gas through aplasma forming CH₃ and H radicals and exposing the surfaces ofair-bridge structures 309 and 312 to the radicals. The CH₃ radicalsreact with the surfaces of air-bridge structures 309 and 312 to make thesurfaces hydrophobic. Alternatively, methane radicals are formed byexposing methane gas to a high frequency electric field.

[0035]FIG. 4 is a block diagram of a computer system suitable for use inconnection with the present invention. System 400 comprises processor405 and memory device 410, which includes conductive structures of oneor more of the types described above in conjunction with FIGS. 1-3.Memory device 410 comprises memory array 415, address circuitry 420, andread circuitry 430, and is coupled to processor 405 by address bus 435,data bus 440, and control bus 445. Processor 405, through address bus435, data bus 440, and control bus 445 communicates with memory device410. In a read operation initiated by processor 405, addressinformation, data information, and control information are provided tomemory device 410 through busses 435, 440, and 445. This information isdecoded by addressing circuitry 420, including a row decoder and acolumn decoder, and read circuitry 430. Successful completion of theread operation results in information from memory array 415 beingcommunicated to processor 405 over data bus 440.

Conclusion

[0036] An insulator for use in high density integrated circuits and amethod of fabricating the insulator has been described. The insulatorincludes a foamed material layer having a surface treated to make ithydrophobic. The method of fabricating the insulator includes forming amaterial layer on a substrate, foaming the material layer to form afoamed material layer, and immersing the foamed material layer in aplasma of methane radicals to make the surface of the foamed materiallayer hydrophobic.

[0037] Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is intended that this invention be limited onlyby the claims and the equivalents thereof.

What is claimed is:

1. A method of forming an insulator comprising: forming a material layerhaving a material layer thickness on a substrate; foaming the materiallayer to form a foamed material layer having a surface and a foamedthickness, the foamed thickness being greater than the material layerthickness; and treating the surface to make the surface hydrophobic. 2.The method of claim 1, wherein forming a material layer having athickness on a substrate comprises: forming a polymer layer on thesubstrate.
 3. The method of claim 1, wherein forming a material layerhaving a thickness on a substrate comprises: applying an aerogel to thesubstrate; spinning the substrate; and curing the aerogel such that,after curing, the thickness is between about 0.6 microns and about 0.8microns.
 4. A method of forming an insulator comprising: forming apolymer layer on a substrate; foaming the polymer layer to form a foamedpolymer layer having a surface and a foamed polymer dielectric constantbetween about 0.8 and about 1.0; and treating the surface to make thesurface hydrophobic.
 5. The method of claim 4, wherein forming a polymerlayer on a substrate comprises: depositing polyimide containing silaneon the substrate.
 6. The method of claim 4, wherein foaming the polymerlayer to form a foamed polymer layer having a surface and a foamedpolymer dielectric constant between about 0.8 and 1.0 comprises: forminga foamed polymer layer having a depth of between about 1.8 and 2.0microns.
 7. The method of claim 4, wherein treating the surface to makethe surface hydrophobic comprises: flowing methane radicals over thesurface.
 8. A method of forming an insulator comprising: forming apolymer layer on a substrate; foaming the polymer layer with asupercritical fluid to form a foamed polymer layer having a surface anda foamed polymer dielectric constant between about 0.8 and 1.0; andtreating the surface to make the surface hydrophobic.
 9. The method ofclaim 8, wherein forming a polymer layer on a substrate comprises:forming a polyimide layer on the substrate.
 10. A method of forming aninsulator comprising: forming a polymer layer having a thickness on asubstrate from a polymer having a silane additive; foaming the polymerlayer to form a foamed polymer layer having a surface and a foamedpolymer layer thickness, the foamed polymer layer thickness is greaterthan the polymer layer thickness by a factor of about between about 2.8and 3.2; and treating the surface to make the surface hydrophobic.
 11. Amethod of forming an insulator comprising: forming a polymer layerhaving a thickness on a substrate from a polymer having a silaneadditive; foaming the polymer layer to form a foamed polymer layerhaving a surface and a foamed polymer layer thickness, the foamedpolymer layer thickness is greater than the polymer layer thickness; andexposing the surface to a gas.
 12. The method of claim 11, whereinexposing the surface to a gas comprises: exposing the surface tomethane.
 13. The method of claim 11, wherein exposing the surface to agas comprises: passing a methane gas through a plasma to form aplurality of methane radicals; and exposing the surface to at least someof the plurality of methane radicals.
 14. A method of forming aninsulator comprising: forming a polymer layer having a thickness on asubstrate from a polymer having a silane additive; foaming the polymerlayer to form a foamed polymer layer having a surface, a cell size, anda foamed polymer layer thickness, the foamed polymer layer thickness isgreater than the polymer layer thickness, and the cell size is less thanabout one-tenth of a micron; and exposing the surface to a gas.
 15. Amethod of forming an insulator comprising: forming an aerogel layer on asubstrate, the aerogel layer having a surface; and treating the surfaceto make the surface hydrophobic.
 16. The method of claim 15, whereinforming an aerogel layer having a surface on a substrate comprises:forming an aerogel layer having a cell size of less than one micron. 17.The method of claim 15, wherein forming an aerogel layer having asurface on a substrate comprises: forming an aerogel layer having a cellsize of less than one-tenth micron.
 18. The method of claim 15, whereintreating the surface to make the surface hydrophobic comprises: exposingthe surface to methane radicals.
 19. The method of claim 15, whereintreating the surface to make the surface hydrophobic comprises: forminga plurality of methane radicals using a high frequency electric field;and exposing the surface to at least some of the plurality of methaneradicals.
 20. A method of forming an insulator comprising: forming anair-bridge structure having a surface on a substrate; and treating theair-bridge structure to make the surface hydrophobic.
 21. The method ofclaim 20, wherein treating the surface to make the surface hydrophobiccomprises: forming a plurality of methane radicals using a highfrequency electric field; and exposing the surface to at least some ofthe plurality of methane radicals.
 22. The method of claim 8, whereinfoaming the polymer layer with a supercritical fluid includes exposingthe polymer layer to a CO₂ supercritical fluid.
 23. The method of claim8, wherein foaming the polymer layer with a supercritical fluid includesexposing the polymer layer to a supercritical fluid with a criticalpressure below about 100 atmospheres and a critical temperature of aboutroom temperature.
 24. The method of claim 8, wherein foaming the polymerlayer with a supercritical fluid includes exposing the polymer layer toa supercritical fluid that is nontoxic and nonflammable.
 25. The methodof claim 8, wherein foaming the polymer layer with a supercritical fluidincludes exposing the polymer layer to a supercritical fluid selectedfrom a group comprising NH₃, NR₃, ROH, H₂O, CO₂, N₂O, He, Ne, Ar, HF,HCl, HBr, BCl₃, Cl₂, F₂, O₂, N₂, CH₄, C₂H₆, C₃H₈, C₂H₄, CO(OCH₃)₂, CF₄,C₂F₄, CH₃F, and C₅H₂F₆O₂.
 26. The method of claim 8, wherein the methodfurther includes depressurizing after exposing the polymer layer to thesupercritical fluid at a rate such that the polymer layer converts tothe foamed polymer layer before substantial diffusion of thesupercritical fluid out of the polymer layer occurs.
 27. The method ofclaim 10, wherein forming a polymer layer includes forming a layer ofparylene containing silane followed by a low temperature bake.
 28. Themethod of claim 10, wherein the method further includes curing thepolymer layer before foaming the polymer layer.
 29. The method of claim14, wherein exposing the surface to a gas includes exposing the surfaceto methane.
 30. The method of claim 14, wherein exposing the surface toa gas includes: passing a methane gas through a plasma to form aplurality of methane radicals; and exposing the surface to at least someof the plurality of methane radicals.
 31. The method of claim 20,wherein treating the surface to make the surface hydrophobic includes:passing methane gas through a plasma forming a plurality of methaneradicals; and exposing the surface to at least some of the plurality ofmethane radicals.
 32. A method of forming a conductive structurecomprising: forming a material layer having a material layer thicknesson a substrate; foaming the material layer to form a foamed materiallayer having a surface and a foamed thickness, the foamed thicknessbeing greater than the material layer thickness; treating the surface tomake the surface hydrophobic; forming channels in the foamed materiallayer; and filling the channels with a metal.
 33. The method of claim32, wherein filling the channels with a metal includes filling thechannels with a metal selected from a group consisting of silver,aluminum, gold, copper, tungsten, and alloys of silver, aluminum, gold,copper, and tungsten.
 34. The method of claim 32, wherein formingchannels in the foamed material layer includes: applying a photoresistto the surface of the foamed material layer; forming patterns for thechannels in the photoresist; and etching the photoresist.
 35. A methodof forming a conductive structure comprising: forming a polymer layerhaving a thickness on a substrate, the polymer layer having a silaneadditive; foaming the polymer layer to form a foamed polymer layerhaving a surface and a foamed polymer layer thickness, the foamedpolymer layer thickness greater than the polymer layer thickness;exposing the surface to a gas; forming channels in the foamed materiallayer; and filling the channels with a metal.
 36. The method of claim35, wherein foaming the polymer layer includes forming a foamed polymerlayer having a thickness of between about 1.8 and 2.0 microns.
 37. Themethod of claim 35, wherein foaming the polymer layer includes forming afoamed polymer layer of polyimide containing silane.
 38. The method ofclaim 37, wherein the method further includes curing a layer ofpolyimide containing silane before foaming the layer of polyimidecontaining silane.
 39. A method of forming a conductive structurecomprising: forming an aerogel layer on a substrate, the aerogel layerhaving a surface; treating the surface to make the surface hydrophobic;forming channels in the foamed material layer; and filling the channelswith a metal.
 40. The method of claim 39, wherein forming an aerogellayer includes applying an aerogel to the substrate; spinning thesubstrate; and curing the aerogel.
 41. The method of claim 39, whereinfilling the channels with a metal includes filling the channels with ametal selected from a group consisting of silver, aluminum, gold,copper, tungsten, and alloys of silver, aluminum, gold, copper, andtungsten.
 42. The method of claim 39, wherein forming channels in thefoamed material layer includes: applying a photoresist to the surface ofthe foamed material layer; forming patterns for the channels in thephotoresist; and etching the photoresist.
 43. A method of forming aconductive structure comprising: forming an air-bridge structure havinga surface on a substrate; and treating the air-bridge structure to makethe surface hydrophobic.
 44. The method of claim 43, wherein treatingthe surface to make the surface hydrophobic comprises: forming aplurality of methane radicals using a high frequency electric field; andexposing the surface to at least some of the plurality of methaneradicals.
 45. The method of claim 43, wherein forming an air-bridgestructure includes fabricating the air-bridge structure using a metalselected from a group consisting of silver, aluminum, gold, copper,tungsten, and alloys of silver, aluminum, gold, copper, and tungsten.46. The method of claim 43, wherein treating the surface to make thesurface hydrophobic comprises: passing methane gas through a plasmaforming a plurality of methane radicals; and exposing the surface to atleast some of the plurality of methane radicals.